"Hey guys, a friend of mine bought some bitcoins around January. His laptop is broken now but he has got an backup on his usb. One .txt file which has his bitcoin adress in it and one .dat file named BTC.dat ."

What programm do I need to download in order to import it back into it to get the coins.

Do yourself a favor, and make a gmail email address for the sole purpose of storing your wallet.dat file. Email the ENCRYPTED wallet.dat file to your new email address, and leave it there forever. You'll be glad you did one day.